The invention relates to a process for the computer-controlled, frontal, alternating sealing of ducts, extending all the way through in the initial condition, pertaining to a honeycomb monolith that can be ceramed, for exhaust gas cleaning systems of internal combustion engines wherein the openings, vacant later on during operation, which openings conduct the oncoming, unpurified gas and, respectively, the outflowing, purified gas of the ducts, are, with the aid of metering heads of a metering device,
first sealed temporarily with a first, hardenable, viscous composition, after the hardening of which the thereafter still vacant duct openings are sealed with a second, likewise hardenable, viscous composition having a permanent effect, and
subsequently the first composition is removed during the course of hardening of the second composition.
Such exhaust gas cleaning systems are finding increasing usage, for example, as soot filters in passenger cars, trucks, and especially in city buses driven by means of a diesel engine. However, exhaust gas cleaning has gained rising significance also in stationary facilities, such as, for example, emergency power units.
The effect of such a filter is based on the feature that the ducts of the honeycomb monolith are sealed alternatingly on both sides in such a way that the exhaust gas is forced to flow through the porous walls between the ducts. In this arrangement, the walls act as filters for the soot, which latter occurs in fine particles. However, the ducts are clogged with soot and other deposits after a certain amount of time. Therefore, such a filter must be repeatedly regenerated, i.e. heated to temperatures at which the retained particles can be oxidized.
The blanks for the honeycombs frequently consist of cordierite or also of sintered glass ceramic, produced in most instances by an extrusion process with subsequent drying and sintering. On account of drying and sintering, the body shrinks, and does this often with deformation. This nonuniform shrinkage results in an incalculable deviation of the actual geometry from the desired geometry of an idealized honeycomb body. Furthermore, even the extrusion step alone can lead to a deformation of the honeycomb structure.
These deviations cause considerable problems in the further processing of the honeycomb monolith, for example when the ducts must be mechanically sealed alternatingly on both sides.
A standard monolith has a diameter of about 14.4 cm. The rough honeycomb dimensions are about 3.6.times.3.6 mm so that there are about 1,400 honeycomb ducts per end face with a hole cross section of respectively 2.4.times.2.4 mm. Consequently, approximately 700 honeycomb openings and thus filter ducts must be sealed alternatingly on each end face. Based on the deviations from the desired geometry displayed by the monolith, it is impossible to seal the 700 openings per end face all at once in one operating step with a nozzle head having 700 nozzles.
A manual sealing of the ducts is not feasible, either, in any way, in view of the thus-incurred high labor cost.
Devices and methods for the alternating sealing of the honeycomb ducts of a monolithic honeycomb body have been suggested in the state of the art.
Attention is invited to U.S. Pat. No. 4,411,856, as one example. This reference proposes the use of a mask completely covering one end face of the monolith, this mask being provided with passages and with peg-like extensions on the side facing the monolith. The pegs are placed into the ducts that are not to be sealed whereas the composition is introduced through the passages in the mask into the end regions of the ducts to be sealed. An attempt is made to take the aforementioned deviation of the monolith from a desired geometry into account by making the mask utilized of an elastic material.
This method may be applicable within limits. There is, though, the problem that the spacing of the above-mentioned pegs to the passages is fixed so that the compensation of the deviations of the monolith from the desired geometry, obtained by the elasticity of the mask, remains restricted to very low values.
EP 0 042 302 B1 discloses a process for the production of a ceramic honeycomb filter from a porous, ceramic honeycomb body with a plurality of ducts which latter extend through the body from one frontal face to the other frontal face, a perforated film being arranged at each frontal face end, the holes of this film corresponding with specific ducts at the frontal face ends; a sealing material is introduced into the ducts through these perforations in the film in order to seal the end zones of these specific ducts, and certain ducts that have not been sealed at the one frontal face end are sealed at the other frontal face end. As a modification of this process, the reference describes that the perforations of the film correspond to those ducts which are not to be sealed so that a heat-volatile material is filled into the ducts which volatilizes during the baking or sintering of the honeycomb, that the films are removed, and that the remaining ducts not filled with the heat-volatile material are sealed with a sealing compound.
Here again, it is very expensive to produce a perforated film, the perforations of which must correspond to the ducts on the frontal face ends. Probably, the film must carry differing perforations for each honeycomb body, even for each frontal face of a single honeycomb body; this precludes an inexpensive, economical manufacturing process.
A procedure is known from the teaching of U.S. Pat. No. 4,329,162 wherein metering heads of a metering device are utilized, with nozzles that are lowered into the ducts to be sealed and introduce sealing compound into the end zone thereof.
However, the reference does not disclose how this is to be done in an economical fashion, with tolerable expense.
A computer can be derived from U.S. Pat. No. 4,557,773 which stores an image of the end face of the monolith, scanned by an image recorder, in a memory and processes this image, and lowers tools into ducts of the scanned end face in correspondence with a fixedly determined operating plan.
Also according to U.S. Pat. No. 4,557,773, it is necessary to cover the end faces with a foil, with great expense of labor.
DE 42 11 787.9, owned by applicant firm, a process is described for the production of a filter in the form of a ceramic honeycomb monolith with a plurality of continuous ducts which latter are sealed alternatingly with a plug at the end faces of the monolith, starting with a blank of the honeycomb monolith of material that can be ceramed, having ducts open on both sides. In this process, zones of ducts are determined on both sides corresponding, within predeterminable tolerances, to the desired geometry of an idealized honeycomb monolith, with the proviso that the zones contain a maximally high number of ducts; metering heads of a metering device having varying numbers of nozzles, which latter are lowered into the ducts to be sealed and introduce the viscous plugging composition into the end zone thereof, are driven and activated by a motor in such a way that, in correspondence with the aforementioned determination of the duct zones, the metering head having the highest number of nozzles is utilized with greatest frequency, and thereafter the metering head having the next-lower number of nozzles is employed, until all provided ducts have been sealed and the thus-prepared honeycomb monolith is subsequently subjected to a ceraming process.
In this method, the sealing compound is introduced into the honeycomb ducts directly by means of the single or multiple nozzle heads after the honeycomb geometry has been ascertained. The nozzles utilized must here correspond approximately to the diameter of the honeycomb ducts in order to fill the duct completely; however, this makes positioning considerably difficult.
Furthermore, the metering device must be adapted exactly to the rheological, chemical, and physical properties of the respective sealing compounds which are often rheologically different.
All of the parts of the facility must be composed of expensive, corrosion-proof materials in order to avoid corrosion and decomposition processes as a consequence of the chemical aggressiveness of the sealing compound.
Moreover, the materials must also be resistant to abrasion inasmuch as the sealing compound frequently contains relatively coarse-grained and, in part, very hard and splintery components.
During each change of the sealing compound or when the facility is shut down, it is necessary to clean the entire system at great expense. This means that the system must first be emptied of all material, causing additional loss of material and production of waste. Subsequently, the entire facility, in the most favorable case, must be washed out by several intermediate flushing steps or, in case of hardened residues, must even be disassembled and cleaned by hand.
In order to obtain homogeneity of the composition, it is furthermore necessary to practice constant agitation, mixing, and recirculation by pumping to avoid settling in the pipelines and unmixing phenomena within the composition.